What drives the arrow of time? How does macroscopic irreversibility arise from microscopic reversibility? What makes entropy increase for closed systems, but decrease in certain open systems?

From the viewpoint of statistical thermodynamics, one can model the evolution of any discrete system by its possible macro states and micro states.

Those macro states having more possible micro states will be more likely to occur, and the macro states having less micro states will be less likely.

Similarly, those macro events caused by more possible micro events will be more likely to obtain, and the macro events caused by less micro events will be less likely.

Therefore, the probabilities of how the past effects the future are determined by the arrangements of the parts making up the micro states and macro states, and similarly the chains of causes constituting the relations between the micro events and macro events.

Apparently time is a progression of events unfolding from the more ordered to the less ordered. However, we know that local order can increase while global order decreases, even if we are unclear as to why. Information and organization can grow; nature and biological evolution are proof of it.

So there is an arrow of time, yet one might think that time is more like a river. (Heraclitus said you could not step into the same river twice.) There is a main flow of the current that carries most everything downstream to disorganization and increasing entropy, but there are eddies here and there that actually increase information and organization.

What enables this to happen? Some say thermodynamic gradients. Some say quantum entanglement. Some say gravity. Some say by the expansion of the universe. Some say dark matter or dark energy. Some say sorting processes.

Can we think of time as being “reversed” in these eddies where information and organization increase locally? No, but it’s an interesting (unscientific) thought.

References and Further Reading:

http://www.informationphilosopher.com/problems/arrow_of_time/

https://en.wikipedia.org/wiki/Arrow_of_time

https://en.wikipedia.org/wiki/Statistical_mechanics

https://en.wikipedia.org/wiki/Entropy

https://en.wikipedia.org/wiki/Introduction_to_entropy

https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

https://en.wikipedia.org/wiki/Entropy_(statistical_thermodynamics)

https://en.wikipedia.org/wiki/Entropy_(classical_thermodynamics)

https://en.wikipedia.org/wiki/Entropy_(order_and_disorder)

https://en.wikipedia.org/wiki/Entropy_(information_theory)

https://en.wikipedia.org/wiki/Sorting

Italo Scardovi / Time and Chance: a statistical hendiadys

https://rivista-statistica.unibo.it/article/viewFile/75/71

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